This invention relates to an apparatus and method for carrying out X-ray fluorescence spectrometry (XRF), and particularly to a portable apparatus, which is able to generate X-ray fluorescence in materials at a distance from the apparatus.
X-ray fluorescence spectrometry is a non-destructive technique for determining the elemental composition of a wide variety of materials. X-ray fluorescence (XRF) is the secondary emission of X-rays at wavelengths characteristic of each element present when a material is irradiated with a primary X-ray beam. In commercially available XRF spectrometers, the bulk sample is usually irradiated directly by X-rays from a sealed tube. The technique is sufficiently sensitive to detect elements which are present at concentrations as low as one or two parts per million. There is, however, a requirement for greater sensitivity in applications in which it is desired to examine small areas on bulk samples or where the sample itself is small. The type of instrumentation required for this technique is sometimes called Micro X-ray Fluorescence Analysis (MXRFA or MXA) apparatus.
Several methods presently exist for MXRFA. Among them is the use of mono-capillary and poly-capillary X-ray focusing optics coupled to standard or microfocus X-ray generating tubes. These suffer from the drawback that samples have to be placed very close to the output of the optic (generally less than 300 microns (11,811.02 microinches). The minimum focal spot generally commercially available with polycapillaries is 28 microns (1,102.36 microinches). This is relatively large and limits the fineness of the resolution with which areas of a sample can be analyzed.
Another method, which presently exists for MXRFA is to use a synchrotron in conjunction with Fresnel lenses. Such apparatus is massive and not portable, although beams having a focal spot of only 1 microns (39.37 microinches) can be achieved, giving greater accuracy in analysis of samples. This method suffers from the disadvantage that synchrotron radiation sources are large fixed facilities, which are not portable and are not available in most laboratories, so cannot be accessed on a routine basis.
A further method of MXRFA, which exists is the use of a synchrotron in conjunction with mono-capillary lenses. Such apparatus is also not portable, and beam sizes are limited to a focal spot of 5 microns (196.85 microinches)-10 microns (393.70 microinches).
It is therefore an object of the invention to provide an apparatus for carrying out X-ray fluorescence spectrometry which is portable yet which is capable of analyzing samples of less than 30 microns (1,181.10 microinches).
According to a first aspect of the present invention there is provided an apparatus for carrying out X-ray fluorescence spectrometry comprising an X-ray generating tube and two paraboloidal X-ray reflecting mirrors, the generating tube having an X-ray source and an X-ray exit window through which X-ray radiation from said source is emitted, the first mirror being aligned on a first axis and positioned in close coupled arrangement adjacent to the exit window, the second mirror being aligned on said first axis and being positioned in spaced apart relationship to the first mirror, the first mirror being adapted to collect diverging X-ray radiation at its first end adjacent to the collecting window and to emit X-ray radiation in a substantially parallel beam at its second end, the second mirror being adapted to collect substantially parallel X-ray radiation at its first end closest to the first mirror and to emit X-ray radiation in a focused beam at its second end.
By using first and second mirrors in this way, the focal spot on the target of the X-ray tube is transferred to the image plane, at unity magnification. The focal spot at the image plane on the sample subjected to fluorescence has a high brightness, and focal spots on the sample of diameter less than 15 microns (590.55 microinches) are possible.
Preferably the first and second mirrors are cylindrical specularly reflecting mirrors. Preferably the first end of the first mirror is positioned between 5 millimeters (0.20 inches) and 50 millimeters (1.97 inches) from the X-ray source.
Preferably the apparatus further comprises a housing containing the first and the second mirrors.
The second mirror may be fixed in position relative to the first mirror.
Alternatively the second mirror may be movable in position relative to the first mirror. The apparatus may further comprise a guide means for guiding the second mirror in a direction parallel to the first axis, and adjustment means for adjusting the spacing of the first and the second mirrors.
The apparatus may further comprise angular adjustment means adapted to allow angular adjustment of the mirror housing with the X-ray generator tube.
Preferably the X-ray generator tube is adapted to produce an X-ray source at the target having a maximum width of less than 50 microns (1,968.50 microinches), more preferably less than 15 microns (590.55 microinches).
According to a second aspect of the present invention there is provided a method of delivering X-ray radiation to a specimen for the purpose of X-ray fluorescence spectrometry using an X-ray generating tube, the generating tube having an X-ray exit window through which X-ray radiation is emitted, the method comprising placing first and second paraboloidal X-ray reflecting mirrors between the exit window and the specimen, using the first mirror to collect diverging X-ray radiation at its first end adjacent to the exit window and to emit X-ray radiation in a substantially parallel beam at its second end, and using the second mirror to collect substantially parallel X-ray radiation at its first end closest to the first mirror and to emit X-ray radiation at its second end to a focused spot on the specimen.
Preferably, the method uses an apparatus according to the first aspect of the invention.